Some vehicles, such as asphalt pavers, comprise independent driving means on opposite sides of the vehicle where the speed of each driving means is separately controlled. Each driving means often comprises a hydraulic pump for driving a motor that is coupled to a crawler or wheel(s) on one side of the vehicle. The crawler or wheel(s) are generally fixed relative to a body of the vehicle. Though the crawler or wheels are permitted to rotate forward and backward, the crawlers or wheels cannot be steered to steer the vehicle in turn. Rather, the relative speed of the two driving means is adjusted to steer the vehicle.
Such vehicles thus have speed controllers; in some cases, there is a separate speed controller—e.g. a joystick—for each side of the vehicle; in other cases, one controller determines the forward speed of the vehicle, and another determines the deviation from straight-ahead, i.e. the desired angular speed or rate of yaw. The latter may be, for example, a steering wheel or a knob. The speed commands for each side of the vehicle are derived from these controllers.
If both sides of the vehicle are not matched, (that is the actual speed on each side is equal) when the respective speed commands are at equal, the vehicle will not move straight ahead, which is a problem for the operator.
Calibration systems and procedures exist, for example for pavers, whereby the operator makes manual adjustments to speed control-system settings. These may be in the form of changes in the position of a trim-pot, or in the value of control parameters such as software control parameters. For example, the operator judges whether the calibration is adequate by fixing his gaze on a distant object, and deciding whether the vehicle is moving toward it directly—as opposed to turning, when the speed controls are set equally. Adjustments to the speed control of one side or the other may be made and the test repeated as necessary.
Such procedures have shortcomings: they are subjective, in that they depend on the operator's perception; they are not always accurate, since they depend on how meticulous the operator is in the performance of the task; and they may be iterative and time-consuming, for example when the adjustments are done by means of trim-pots they may involve adjusting one or more trim-pots and testing, then adjusting again and testing again, and so on; this makes the procedure quite inconvenient and costly, especially if the trim-pots are not easily accessible.
Therefore there is needed a method and system to facilitate calibration of both sides of the vehicle so that it will track straight when the speed commands are equal. In addition, there is needed a method and system to enable the vehicle to track straight automatically, even when the calibration is not accurate. A solution that is reliable, easy to implement, and relatively inexpensive is highly desired.